(1) Field of the Invention
The invention is in the general field of semiconductor doping, more specifically neutron transmutation doping (NTD).
(2) Description of the Prior Art
The most widely used method for the production of silicon ingots for the semiconductor industry is the Czochralski (CZ) method. In the CZ method, super pure silicon is melted in a silica crucible, a single crystal silicon seed is lowered into the melt and then slowly withdrawn. Some liquid silicon is withdrawn along with the seed, because of surface tension, and then solidifies in the slightly cooler environment above the melt. Depending on rate of withdrawal and other factors, high quality silicon single crystals with diameters up to about 12 inches and lengths to about 40 inches are routinely produced in this way.
In general, the resistivity of CZ crystals is higher than desired for semiconductor manufacturing purposes and a dopant, such as phosphorus, may be added to the melt to lower the resistivity. A number of problems are associated with this method of controlling the resistivity of silicon ingots, notably those of non-uniformity. In general, it is found that the doping level can vary substantially both along the length of the ingot as well as along its cross-section. A certain amount of improvement in the latter distribution has been achieved in the Magnetic Czochralski (MCZ) method wherein crystal growth takes place in the presence of a longitudinal magnetic field (typically between about 500 and 4,000 Gauss). Since molten silicon is an electrical conductor, convection currents in it will tend to follow the lines of magnetic force. However, although the MCZ method offers some improvement, the basic problems of non-uniform distribution of dopant remain.
Silicon has an atomic number of 14 and an atomic weight of 28. However, naturally occurring silicon is not entirely made up of the Si.sup.28 isotope. It turns out that Si .sup.29 is present at a concentration of about 4.7 atomic % and Si.sup.30 is present at a concentration of about 3.1 atomic %. Additionally, it turns out that Si.sup.30, when bombarded by thermal neutrons, is transmuted to phosphorus p.sup.31 (atomic number 15). Since the desired level of phosphorus doping is well below the 3.1 at. % of the already present Si.sup.30, it is apparent that a limited amount of neutron bombardment of naturally occurring silicon, will result in the introduction of phosphorus dopant into the silicon. Such phosphorus dopant will be uniformly distributed and will also be in substitutional position in the lattice where it can act as a donor.
This method of doping CZ and MCZ silicon ingots has been described in the prior art by, for example, Takasu et al. (U.S. Pat. No. 4,910,156 March 1990). The neutron sources available outside the nuclear weapons industry all provide slow (thermal) neutrons (ideally having energies around 0.025 eV). These sources include pool type reactors, producing neutrons directly, proton cyclotrons, and proton linear accelerators. The proton generators are in general more convenient to use as the proton stream can be magnetically steered. The fast protons that these generators produce are converted to fast neutrons by causing them to pass through a material such as beryllium, tungsten, or uranium. These fast neutrons are then slowed to become thermal neutrons by surrounding moderators, such as heavy water (D.sub.2 O).
While neutron transmutation doping (NTD) is an ideal way to achieve uniform phosphorus doping, CZ silicon crystals doped by the NTD method normally do not end up with the expected results. Oxygen interstitials are commonly present within CZ and MCZ silicon crystals as an unintended impurity, which actually comes from the silica crucible itself. Oxygen interstitials are desirable for the manufacture of microelectronic devices owing to several salient features, such as facilitating intrinsic gettering of VLSI process-induced impurities and hardening substrate material against thermal stresses. However, excessive oxygen precipitation (i.e. silica formation) tends to be induced by neutron bombardment of the NTD method and this causes the resulting resistivity to be quite unpredictable. In some cases, total oxygen precipitation even occurred. Excessive oxygen precipitation within the silicon substrate leads to undesirably short minority carrier lifetimes and thus unacceptably large leakage currents of microelectronic devices under reverse bias.
In order for the NTD method to be successfully applied to CZ silicon crystals, the working recipe of the present invention must be known. This covers the initial oxygen content of the CZ silicon crystal, neutron energy spectrum, and neutron fluence (total accumulated number of impinging neutrons per sq. cm.). Without such know-how, the resulting NTD CZ silicon crystals are of unpredictable quality. It is suspected that, due to such a lack of knowledge, Takasu et al. (U.S. Pat. No. 4,910,156 March 1990) proposed using several non-traditional infrared probing techniques for NTD CZ wafer quality control and assurance. NTD wafers with low infrared transparency failed to become prime wafers. However, with the working recipe of the present invention, good product NTD CZ prime wafers can be assured and the non-traditional infrared probing is unnecessary. Also, in the claims made by Takasu et al. the allowed maximum fast neutron fluence was specified based upon its relation to the leakage currents of reverse-biased circuits built on their production NTD wafers. Nonetheless, according to experiments guided by the recipe of the present invention, the fast neutron vs. leakage current (in arbitrary units) relation of Takasu et al. was simply incorrect. Additionally, the partition between fast and slow neutrons is rather vague (in Takasu et al.) but this turns out to be a very important parameter (as is taught by the present invention).
We should also note that NTD has been used to dope individual silicon wafers, as opposed to full ingots. See for example Groves et al. (U.S. Pat. No. 5,212,100 May 1993) but the same problems relative to oxygen precipitation will still apply. The present invention, which we will describe in detail below, has been able to use NTD in a manner that not only leads to uniform phosphorus doping but also successfully avoids the very serious excessive oxygen precipitation problem.